1. Field of the Invention
The present invention relates to torque wrenches and more particularly to such a torque wrench having a numeric display for displaying a preset maximum torque and an amount of torque being exerted so that the wrench may stop tightening a thing (e.g., bolt, nut, etc.) automatically when the amount of torque has reached the preset maximum torque.
2. Description of Related Art
Torque wrenches are well known. For example, a prior torque wrench is shown in FIG. 1. The wrench comprises an elongated, hollow handle 1, an enclosed box portion 2 at a forward end of the handle 1, the enclosed box portion 2 including a head 2a and an elongate bar 2b extended into the handle 1, a trigger member 3 at a rear end of the bar 2b, a spring 4 having a forward end biased against a rear end of the trigger member 3, a rod 5 having a forward end biased by a rear end of the spring 4, and a rotatable cap 6 having internal threads adapted to secure to an externally threaded section at a rear end of the handle 1. A maximum torque can be set as indicated on a dial by rotating the cap 6 about the handle 1. Also, the rod 5 moves forward to compress the spring 4 which in turn pushes the trigger member 3 to engage with the rear end of the bar 2b. In use, the trigger member 3 may trip (i.e., the bar 2b is disengaged from the trigger member 3) immediately with an audible click being produced when the amount of torque being exerted by the wrench has reached the preset maximum torque. However, such mechanism has the drawbacks of being low in precision and trouble prone.
A Taiwanese Patent Published No. 84,211,488 disclosed an electronic torque wrench as shown in FIGS. 2 and 3. The wrench comprises an elongated, hollow handle 7, an enclosed box portion 8 at a forward end of the handle 7, the enclosed box portion 8 including a head 8a and an elongate bar 8b extended into the handle 7, a trigger member 9 at a rear end of the bar 8b, a spring 10 having a forward end biased against a rear end of the trigger member 9, a rod 11 having a forward end biased by a rear end of the spring 10, a sleeve 14 having an internally threaded section at a rear end of the handle 7, a rotatable cap 13 including a hollow barrel 12 having external threads inserted through the sleeve 14 into the rod 11 so as to threadedly secure the barrel 12 and the sleeve 14 together, and a torque measurement device 5 mounted on a rear portion of the handle 7, the torque measurement device 5 including a conversion circuit, a sensor formed of variable resistor, the sensor having a sliding member 16 inserted through an elongate groove 7a of the handle 7 into the rod for coupling, and a numeric display. A maximum torque can be set as detailed below. Rotate the cap 13 about the handle 7 to threadedly advance the barrel 12 for pushing the rod 11 and thus the sliding member 16 forward. Also, the spring 10 is compressed. And in turn the trigger member 9 is pushed by the spring 10 to engage with the rear end of the bar 8b. Electrical resistance of the sensor is thus changed as the sliding member 16 moves. Eventually, the preset maximum torque is shown numerically on the display after converting the electrical resistance change into digital signal by the conversion circuit. Likewise, in use the trigger member 9 may trip when the amount of torque being exerted by the wrench has reached the preset maximum torque. However, such mechanism still has the drawbacks of having complicated components and poor precision if the movement of the sliding member 16 is not smooth, elasticity of the spring 10 is fatigued, or a contact between the trigger member 9 and the rear end of the bar 8b is loosened. Above drawbacks may compromise the desired effect of automatically tripping the trigger member 9 when the amount of torque being exerted by the wrench has reached the preset maximum torque. This is because the amount of torque being exerted by the wrench may be not equal to the preset maximum torque (i.e., it changed) when the automatic trip occurs. Thus, the need for improvement still exists.